It's interesting to note that DSCH considered calling the Michelangelo songs his 16th Symphony.I like the 14th a lot, but have difficulty envisioning it as a symphony in the way I can with Das Lied von der Erde or DSCH's own 13th. Because it consists of relatively short passages it feels like it has more in common with the orchestral versions of the later song cycles (Six Poems by Marina Tsevayeva op.143a and Suite on Verses of Michelangelo Buonarroti op.145a), not least because the themes of death, despair and resignation seem to run through them all.
If he had done so, it would certainly have given this masterpiece the much needed exposure (since it would be included in complete symphony sets). I'm sure the 2nd and 3rd would be completely obscure works now if they had been called "symphonic cantata" or something and not included in the official list. Now we're saddled with literally dozens of recordings of these mediocre works (which DSCH grew to dislike himself), while you can count recordings of the Michelangelo songs on the fingers of one hand.
On the other hand, there's something comforting in the perfect symmetry of 15 symphonies and 15 string quartets... (not to mention the pairs of piano sonatas, piano trios, piano, violin and cello concertos).